The present invention relates to binder compositions for bonding particulate materials. The invention has particular utility in the foundry industry, for forming bonded particulate articles, including foundry moulds and cores, and other refractory articles for use with hot molten metal, e.g. linings and feeder sleeves, including insulating, exothermic, and duplex (i.e. insulating and exothermic) sleeves.
The formation of foundry moulds and cores from bonded particulate refractory material, e.g. sand, is very well known. It is also very well known to form other refractory articles such as ladle linings, feeder head linings, feeder sleeves and the like, from bonded particulate materials. A feeder sleeve provides a reservoir for molten metal and enables the molten metal to remain molten longer than a casting in which it is being employed. The feeder sleeve thus enables the molten metal to continue to feed the casting as it solidifies, providing for a sound and strong casting. Refractory articles, such as linings and feeder sleeves consequently are often formed from insulating materials, to reduce heat losses. Some applications (such as feeder sleeves) involve the use of consumable insulators whilst others require insulators that are durable and able to repeatedly cycle through a range of temperatures. High grade, low density insulators (typically 0.5 g/cc) are known and are based on ceramic fibre. High density products, based on silica, typically have an open porous structure.
Feeder sleeves are produced by a variety of methods, including the resin bonding of waste silaceous materials such as so-called xe2x80x9cflyash floatersxe2x80x9d (sometimes known by the trade marks xe2x80x9cExtendospheresxe2x80x9d or xe2x80x9cCenospheresxe2x80x9d). Foundry moulds and cores are often produced by the resin bonding of silica and/or other sand. Resin bonding is generally employed because, when the sleeves, moulds or cores are gas cured in a pattern box, the resin enables good strength and dimensional accuracy to be achieved. However, in the presence of molten metal the resins employed normally generate considerable amounts of fumes and gases. In some circumstances, this fume and gas is absorbed by the molten metal, leading to a deterioration in its quality. The fume problem is particularly problematic in the casting of low temperature alloys, for example those including aluminium where there is insufficient molten metal heat to burn the resins, but sufficient molten metal heat to volatise the components as smoke and fume. It would be advantageous if bonded particulate refractory articles, including moulds, cores and feeder sleeves, could be produced to good dimensional accuracy but without the problem of fume and smoke generation.
In a first aspect the present invention provides a method of producing a bonded particulate material comprising the steps of:
combining an alkali with a particulate metal oxide that is capable of forming a metalate in the presence of the alkali; and
drying the particles after a portion of each metal oxide particle has formed the metalate, in a manner such that an unreacted particle core remains after drying.
By maintaining a metal oxide core of each metal oxide particle in the resultant bonded particulate material, a refractory and/or insulating function can generally be provided and yet high dimensional stability and accuracy can normally be achieved. Also, a high degree of bonding between adjacent metal oxide particles can generally be achieved because the exterior surface of the metal oxide particles typically xe2x80x9cdissolvesxe2x80x9d, thus enabling a bond to form between adjacent metal oxide particles, and which bond xe2x80x9csolidifiesxe2x80x9d after drying.
When the term xe2x80x9cmetalxe2x80x9d is used in the present specification it is intended to include quasi metals such as silicon. When the expression xe2x80x9cmetal oxidexe2x80x9d is used, it is used in relation to a solid metal oxide that is typically capable of use as a refractory material, an insulating material, a construction material, or other bonded particulate material. When the expression xe2x80x9cparticulate materialxe2x80x9d is used herein, it includes within its scope fibrous material and/or granular material and/or powder material and/or fines etc. The term xe2x80x9cmetalatexe2x80x9d is used herein to refer to oxo anions (also known as xe2x80x9coxyanionsxe2x80x9d) which may be considered as being formed by the co-ordination of oxide O2xe2x88x92 ions with metal (including quasi metal, such as silicon) cations to form metal-and-oxygen anions, possibly including hydroxide groups, especially under alkaline conditions. These are the normal species in aqueous solution, however. their exact structures are often complex rather than simple discrete species, with typical examples including silicates, titanates, aluminates, zincates, germanates, etc. Such metal-and-oxygen anions (metalates) are then associated with alkali metal cations (such as Na+ or K+) from the alkali.
Most typically the alkali is in the form of an aqueous solution, such that xe2x80x9cdryingxe2x80x9d involves driving off water from the mixture of the metal oxide and the alkali solution. However, if the reaction were conducted in the gas or molten phase, xe2x80x9cdryingxe2x80x9d would mean adjusting the conditions to cause the reaction between the metal oxide and the alkali to stop.
Metal oxides that are preferably employed (and which typically function as a binding material) include silica fume, fine alumina, fine titania, zinc oxides etc (the use of xe2x80x9cfinexe2x80x9d referring to a fine particulate form of the oxide). These materials readily form a metalate in the presence of an alkali solution.
Preferably these types of metal oxides function as a xe2x80x9cbinderxe2x80x9d in the insulator produced in accordance with the invention.
The metal oxide can also be a waste siliceous material, such as flyash, flyash floaters (FAF) or other oxidisable waste oxide; thus, a valuable product can be produced using waste material. (Flyash floaters are hollow microspheres of silica and/or aluminaxe2x80x94they normally comprise aluminosilicate, possibly with other constituents.) A variety of other metal oxides can be employed. For example, silica sand, bauxite, alumina, perlite, etc can be employed. However, usually these latter materials (ie. including FAF) constitute a xe2x80x9cfillerxe2x80x9d component of the bonded particulate material, and form the xe2x80x9cbulkxe2x80x9d of the bonded particulate material rather than providing the major binding function. The filler, or a combination of fillers, is then typically used in conjunction with and bonded by a binding material (as defined above). It should be appreciated, however, that either metal oxide binders or fillers can be used on their own to form the bonded particulate material. Also, some filler employed in the present bonded particulate materials may not have a reactive oxide consistency and hence may only form a relatively weaker bond with a binding material. Preferably when non-oxide fillers are employed they can still bind with a metalate, such as a silicate, aluminate, titanate, zincate etc.
Preferably the alkali is a solution produced from a strong alkali such as sodium hydroxide, potassium hydroxide, lithium hydroxide etc. Sodium hydroxide is most preferred because of its relative abundance and low cost.
In a preferred variation of the method, the binder material is premixed with the alkali solution prior to mixing the filler material therewith. When the alkali solution is based on sodium hydroxide, employing a premix can minimise the amount of alkali solution required. Because sodium acts as a flux in bonded particulate materials, it is desirable to minimise its presence in the resulting bonded particulate material, and it has been observed that the formation of the premix assists in reducing the quantity of sodium present in the resulting bonded particulate material.
Preferably the drying step is conducted in a microwave oven or urn. Microwave radiation has been observed to be an expedient way of achieving drying and forming a bond, thus maintaining a metal oxide core. However, conventional convection and radiation ovens and urns can be employed as can dielectric heating. Additionally or alternatively, a heated core box may be used and/or the drying may be by means of an applied vacuum. The drying, therefore, is preferably by means of heating, by convection and/or conduction and/or radiation (microwave and/or infrared radiation), and/or by means of evaporation, preferably by the use of reduced pressure, i.e. the application of a vacuum or partial vacuum.
In some preferred versions of the invention, there is a curing step prior to the drying step, i.e. the metalate may be partially or completely hardened prior to being dried. This has the advantage of increasing the strength of the particulate article, lessening the possibility of distortion or damage to the article, prior to the drying step. The hardening step may be by means of reaction with carbon dioxide, for example. Advantageously, an atmosphere of carbon dioxide gas may be supplied to the particulate article, for example in a core box or similar in which the article has been formed. The hardening step is generally facilitated if the premix (of binder material and alkali solution) has been aged (and especially if at least some dehydration has occured), for example by being allowed to stand for a period of time (preferably at least 6 hours) subsequent to its preparation and/or the premix has been exposed to microwave radiation.
Formulations having a relatively higher binder content were observed to have high storage strengths (ie. cured strength), but lower fired strength (ie. thermal shock resistance). As such, these formulations preferably are used in applications such as feeder and riser sleeves, and linings.
Formulations having a relatively lower binder content were observed to have higher fired strength, and lower cured strength. Such formulations preferably are used as refractory insulators, such as refractory bricks.
Bonded particulate materials can also be produced having various densities. Higher density materials are generally suitable for use as construction elements, having a lesser insulating function.
In a second aspect the present invention provides a bonded particulate material formed from a plurality of bonded metal oxide particles, wherein each particle has a metal oxide core, surrounded by a metalate layer.
Such a bonded particulate material is typically formed by the method of the first aspect of the invention.
In a third aspect, the present invention provides a binder for bonding a particulate material, comprising:
(a) a particulate metal oxide that is capable of forming a metalate in the presence of an alkali;
(b) an alkali; and
(c) water.
The particulate metal oxide preferably comprises silica, more preferably silica fume. The alkali preferably comprises sodium hydroxide and/or potassium hydroxide.
The alkali is preferably present in an amount of 3-50 weight %, the particulate metal oxide is present in an amount of 10-70 weight %, and the water is present in an amount of 30-70 weight %, based upon the total weight of the binder. More preferably, the alkali is present in an amount of 3-25 weight %, the particulate metal oxide is present in an amount of 20-55 weight %, and the water is present in an amount of 40-60 weight %, based upon the total weight of the binder.
According to a fourth aspect, the present invention provides a composition for forming a bonded particulate article, comprising:
(a) a binder according to the third aspect of the invention; and
(b) a refractory particulate material.
In some-embodiments of the invention, the refractory particulate material and the particulate metal oxide may be one and the same material, i.e. the xe2x80x9cbulkxe2x80x9d or xe2x80x9cfillerxe2x80x9d material may comprise part of the binder composition for bonding itself together. Additionally or alternatively, the particulate metal oxide may be a different material to the refractory particulate material, included as a separate component which is part of a binder composition. The refractory particulate material preferably comprises silica and/or alumina and/or aluminosilicate (e.g. in the form of hollow microspheres).
A fifth aspect of the invention provides a bonded particulate article formed from a composition according to the fourth aspect of the invention. Examples of bonded particulate articles according to the invention include: foundry moulds; foundry cores; feeder sleeves (insulating, exothermic and/or duplex sleeves); linings (e.g. furnace linings, ladle linings, tundish linings, etc.); flow controllers (for molten metal); strainer cores; strainer sleeves; tundish starter tubes; substantially any refractory article for use with molten metal.
Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, with reference to the following non-limiting examples.
Initial Experimentation
Initial experimentation sought to produce a riser sleeve (also known as a feeder sleeve) with the same dimensional accuracy but without the fume problem of a resin bonded insulator. The experiment focussed on the use of sodium silicate and flyash floaters (FAF).